One of the biggest things Photoshop can do that GIMP can't is CMYK separations: if that's important to you, GIMP is not what you want.

Outside of that, it's up to personal preference and price, IMHO. Most of the graphic design folks I know swear by PShop (and swear AT GIMP), but a lot of that may very well be snob factor. They're both good programs--not quite identical, but close enough that for many purposes, the same folks can use them and get the same results.

One other factor that I have seen is differences in the user interfaces: changing from one to the other (no matter which direction you go can be aggravating. If you're used to one, and don't need a specific feature the other offers, I'd stick with what you already have that works.

EDIT: The above is offered from a non-graphic artist computer geek. I do play around with GIMP a bit, but I'm strictly an amateur. However, I did at one time work for a company that contracted with Adobe, so I've played with both.